﻿764 
  

  

  Prof. 
  R. 
  W. 
  Wood 
  on 
  Talhofs 
  I 
  

  

  Tinqes 
  

  

  of 
  the 
  plate. 
  This 
  may 
  be 
  done 
  bj 
  taking 
  the 
  limited 
  region 
  

   of 
  the 
  spectrum 
  DE 
  (fig. 
  2). 
  

  

  Fig. 
  2. 
  

  

  'kf 
  

  

  rlate 
  correctly 
  

   placed. 
  

  

  ■■(\ 
  

  

  > 
  Plate 
  on 
  wrong- 
  

   side. 
  

  

  Assume 
  it 
  to 
  be 
  of 
  infinite 
  purity, 
  and 
  consider 
  that 
  the 
  

   narrow 
  regions 
  at 
  D, 
  A, 
  and 
  E 
  remain 
  single 
  when 
  the 
  re- 
  

   tarding 
  plate 
  is 
  introduced, 
  while 
  those 
  at 
  B 
  and 
  C 
  become 
  

   double, 
  in 
  the 
  manner 
  previously 
  described. 
  Furthermore, 
  

   consider 
  that 
  they 
  also 
  become 
  double 
  when 
  the 
  plate 
  is 
  

   introduced 
  on 
  the 
  wrong 
  side, 
  as 
  is 
  the 
  case 
  when 
  we 
  use 
  

   homogeneous 
  light. 
  I 
  have 
  made 
  the 
  calculation 
  for 
  the 
  

   wave-lengths 
  D, 
  B, 
  A, 
  C, 
  E, 
  and 
  also 
  a 
  number 
  of 
  inter- 
  

   mediate 
  values, 
  by 
  drawing 
  the 
  spectrum 
  on 
  coordinate 
  

   paper, 
  and 
  calculating 
  what 
  happens 
  to 
  each 
  element, 
  as 
  a 
  

   result 
  of 
  the 
  retardation. 
  The 
  lines 
  between 
  those 
  which 
  

   are 
  lettered 
  become 
  double, 
  the 
  components, 
  however, 
  are 
  of 
  

   unequal 
  intensity 
  and 
  are 
  un 
  symmetrically 
  placed 
  with 
  respect 
  

   to 
  the 
  original 
  position 
  of 
  the 
  line. 
  With 
  the 
  retarding 
  plate 
  

   in 
  the 
  right 
  position 
  we 
  find 
  that 
  the 
  elements 
  of 
  the 
  spectrum 
  

   are 
  pushed 
  together 
  into 
  the 
  position 
  of 
  the 
  lines 
  D, 
  A, 
  and 
  

   E, 
  forming 
  the 
  bright 
  Talbot 
  fringes 
  (see 
  fig. 
  2). 
  

  

  In 
  the 
  case 
  of 
  the 
  incorrect 
  position, 
  the 
  regions 
  of 
  the 
  

   spectrum 
  between 
  AB, 
  AC, 
  &c., 
  instead 
  of 
  being 
  squeezed 
  

   together 
  are 
  pulled 
  out, 
  the 
  intensity 
  distribution 
  being 
  as 
  

   shown 
  in 
  fig. 
  2 
  (superposition 
  of 
  3 
  lower 
  figures). 
  It 
  is 
  

   clear 
  that 
  no 
  dark 
  bands 
  appear 
  in 
  this 
  case, 
  and 
  yet 
  at 
  any 
  

   point 
  in 
  the 
  spectrum 
  we 
  shall 
  find 
  in 
  general 
  a 
  number 
  of 
  

   different 
  wave-lengths, 
  as 
  a 
  result 
  of 
  the 
  interference 
  which 
  

   we 
  have 
  assumed 
  to 
  take 
  place. 
  It 
  seems 
  to 
  me, 
  therefore, 
  

   that 
  we 
  can 
  account 
  for 
  the 
  failure 
  of 
  the 
  bands 
  to 
  appear, 
  

  

  